This invention relates to signal amplification frequency-conversion apparatus and more particularly to those of the type including a common or single active component.
Recently, with expansion of the operation frequency range, increase in output and reduction in cost of high-frequency use transistors, such as silicon bipolar transistors and GaAs field-effect transistors, attempts are being made to realize microwave transmitters/receivers in a wholly solid-state form. Such form of transmitter/receiver will obviously be advantageous over the conventionally one, of the travelling-wave tube type, in reliability, cost performance and power consumption.
In the transmitter section of a heterodyne microwave transmitter/receiver for relay use, the transmission frequency-converter generally includes a frequency mixer at which the intermediate-frequency signal and the local signal are frequency-converted to a transmission signal frequency, and a transmission power amplifier section where the transmission signal is power-amplified for transmission. Further, in a reception frequency converter such as used in the receiver section of the transmitter/receiver of the type described or generally in a heterodyne microwave receiver unit, the received signal as leaving the branching filter is amplified by a high-frequency preamplifier and then, at a frequency mixer, is mixed with the local-oscillation signal and frequency-converted into an intermediate frequency. Such transmission and reception frequency converteres serve the functions of signal amplification, frequency mixing and local oscillation and, for reduction in size and cost, it has previously been proposed to make use of a single or common active component to serve both the local oscillation and signal amplification functions.
The amplification-oscillation circuit including a common active component, however, has been problematic because steady state of oscillation is established in the saturation range of the active component. Specifically, letting A be the amplification factor of the active component and .beta., the feedback factor by the feedback circuit, it is required that, to maintain the oscillation once started, the sum of the phase component (Ap) of the amplification factor and the phase component (.beta.p) of the feedback factor is zero and .vertline.A.beta..vertline.&gt;1, and that, in the steady state, Ap+.beta.p=0 and .vertline.A.beta..vertline.=1 are satisfied. In other words, in positive feedback, as long as .vertline.A.beta..vertline.&gt;1, the operating point of oscillation shifts into the saturation region of the active component as the oscillation starts to develop and the amplification factor. A starts to decrease with saturation of the active component until the oscillation is stabilized at the saturation point, where .vertline.A.beta..vertline.=1.
With the operating point of oscillation thus lying in the saturation region of the active component, it will be noted that the signal to be amplified and the oscillation output markedly interfere with each other in cases where a single active component or a single active-component circuit unit is used in common for oscillation and amplification purposes. Such interference causes increase of the intermodulation distortion. Further, the level of oscillation output, which depends upon the saturation level of the active component, becomes less stable, widely varying with the level of signal to be amplified, as the latter approaches the saturation level of the active component. Such unstable oscillation output is inappropriate as a local oscillation output for reception or transmission use.
A transmitter/receiver apparatus, designed to solve such problems of intermodulation distortion and oscillation output level and including a single travelling-wave tube amplifier used in common for amplification and oscillation, has been proposed by T. Kawahashi et al., as seen in the article entitled "Solid-State 4 GHz 1200-Channel System," IEEE International Communications Conference, Philadelphia, June 1966. The apparatus proposed includes a nonlinear circuit, called "local power control (LPC)," inserted in the feedback loop forming an oscillation circuit for the purpose of stabilizing the oscillation output. As apparent, this apparatus is so arranged that the oscillation is effected in the unsaturated region of the amplifier, in order to avoid such problems as intermodulation distortion. Also, in the apparatus, the frequency mixing is effected by mixing the oscillation output extracted from the oscillation loop by means such as a directional coupler with the received signal.
The apparatus proposed by T. Kawahashi et al. is advantageous in that use is made of a single amplifier for both amplification and oscillation purpose but is complicated in structure and uneconomical as it includes an LPC circuit especially provided for stabilization of the oscillation output and oscillation in the linear region of the amplifier and as the frequency mixer is provided exteriorly of the oscillation loop.